Subject: I lost my socks.
Gee, I lost my socks today blah blah blah blah
Comment: (no subject) Bummer
E-mail subject: Live journal comment re: "I lost my socks" [or something similar; I know the from field lists "username - lj comment", but you get the gist]
Drawbacks: Parsing (?) any user HTML out of the subject. This could be harder than it sounds.
Benefits: An end to the "reply to your post/reply to your comment" e-mails (for the most part).
Cause, y'see, I put subjects on most of my posts so I can find them on the view-by-month index. But most of the comments that come in are sans subject.